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No description available.

No description available.

The work described in this thesis comprises an investigation of the reaction 37Al('),p)26Mg and the reaction 7Li(p/7)8Be*. Measurements were made of the cross-sections for the 27Al(f,p)3 Mg reaction, leading to discrete excitation states of the residual nucleus. These measurements were carried out using a quasi-monochromatic photon beam obtained by the photon tagging technique. Cross-sections are presented for photon energies from 47 to 64 MeV and for nucleon emission angles between 60° to 120°. The general features of the data are: i) angular distributions which are forward peaked, ii) cross-sections which decrease exponentially with photon energy. Comparison with the predictions of the direct knockout model indicate that the (f,p) cross sections may be explicable assuming no other process contributes. A comparison of the results from the present work with those of a recent27 Al(e,e'p)26Mg experiment, indicates that the (f,p) and (e.e'p) reactions excite the same residual states (g.s 0+, 1.81 MeV 2+, 2.94 MeV 2+, 4.35 MeV 2+ and 7.86 Mev) in similar relative proportions and exhibit similar functional dependencies on momentum transfer. The 90° yield was measured for proton capture to the region of the 2+ isospin-mixed pair of levels in 8Be* using the reaction 7Li(p,f)8Be* -> 2a for incident proton energy 25 MeV. The measurements were made at the SERC Nuclear Structure Facility, Daresbury, U.K. Gamma-rays were detected by a 25 cm x 35 cm NaI(Tl) detector and two alpha particles were detected by 50 mm x 50 mm silicon strip detectors. Two alpha particles and f-rays were detected all together in a triple coincidence. This new datum is shown to be more consistent with a resonance type reaction mechanism than the previously proposed direct capture process.

Samples based on the barium fluorobromide (BaFBr) host lattice were prepared in a pure form and doped to 0.5mol% with KF, KBr, SrF₂, SrBr₂ and with several rare earth chlorides (EuCl₂, LaCl₃, CeCl₃ and SmCl₃). Compounds with stoichiometric variations on the pure and europium doped barium fluoroiodide (general formula BaF₁I_1-x(:Eu²) where x = 0, 0.25, 0.50, 0.75 or 1) were also synthesised. In addition, the alkali halides NaBr, KBr and RbBr were doped with approximately 0.5mol% of both EuCl₂ and InCl. Emission generally occurred at wavelengths between 360nm and 450nm for these materials. None of them had room temperature emission bands that were as intense and narrow as that of BaFBr:Eu²⁺, because they do not have electronic transitions that are so closely matched to the electron/hole recombination energy as the 4f⁶ 5d¹ → 4f⁷ transition in Eu²⁺. TL glow curves showed a broad high temperature peak which arose as a result of charged defect migration and a variety of lower temperature peaks that were due to recombination from defect states within the band gap. The F(F') centre was matched to the glow curve peak t 400K and the F(Br^-) centre to the lower temperature peak at 370K in a variety of mixed stoichiometry BaFBr-type compounds. It was also observed that a small amount of oxygen within the lattice appeared to enhance the intensity of a couple of the low temperature glove curve peaks, but too much oxygen had the opposite effect and these peaks were quenched. The time dependence of the intensity of PSL at timescales of the order of 100 - 6000 seconds was found to obey a power law decay from 100K to 300K. This could be modelled using random walk kinetics for the migration of the charged defect species. Upon irradiation and photostimulation at low temperatures an increase in the initial intensity of the luminescence and of the magnitude of the gradient of the linear plot was observed. This suggested that more change was stored at low temperatures and that new photostimulable centres were able to form more easily.

In this thesis we present an experimental investigation of the mechanism of storage and stimulation processes in the x-ray storage phosphors BaFBr:Eu and related materials. Such materials are used in radiography and diffraction equipment to image X-rays, but the factors that govern the trapping and release of the image is still unclear. We used photostimulated luminescence (PSL), spontaneous luminescence (SL), and magnetic measurements to study the formation and release of charged defects of a series of pure and doped materials isomorphous with BaFBr. Samples of the materials BaFX (X = Cl, Br, I) and in both a pure form and doped with the rare earth ions La, Ce, Sm, Eu, Gd, Tb, Dy, Tm and Yb were prepared, together with SrFX:Eu (X = Cl, Br, I) and the non-stoichiometric samples BaF<SUB>1-x</SUB>Br<SUB>1+ </SUB>(x = -0.2, -0.1, 0, 0.1, 0.2) and samples doped with various amounts of oxygen. We also prepared the isomorphous compounds LaOX (X = Cl, Br, I). We designed and built a variable temperature photostimulation and detection system to study the PSL and SL of these materials. The emission spectrum of BaFBr samples doped with most rare earth ions was the same as the pure host material, the exceptions being the dopants Ce, Eu and Yb which showed evidence of transitions involving ions not in the trivalent state. Samples doped with Eu gave the most intense emission spectra due to the correspondence between the energy of recombination of electrons and holes in the host lattice, and the 4f<SUP>6</SUP>5d - 4f<SUP>7</SUP> transition of Eu<SUP>2+</SUP>. Emission spectra of the pure host materials show trends that correspond to trends in trap depths for different lattices.

No description available.

Transition metal clusters were generated by laser vaporisation in the throat of a pulsed supersonic expansion. Detection was accomplished by pulsed laser photoionisation in the source of a time-of-flight mass spectrometer (University of Edinburgh), or by laser-induced fluorescence (LIF) using a ring dye laser (National Research Council of Canada, Ottawa). A variety of metal cluster systems from the first and second transition periods were probed using laser photoionisation time-of-flight mass spec-trometry. Resonant two photon ionisation (R2PI) spectroscopy was performed on the A «— X system of CuAg, and the A state conclusively demonstrated tp have 0"*" symmetry. Rotational analysis yielded T"C =2.3664(71) X and re =2.508(14) X for 63Cu107 Ag. The collision-free lifetime for the v = 0 level of the A state was determined to be 32 ± 2 ns. Congested ultraviolet band systems lying at twice the A «— X transition energy were probed using vibronic R2PI spectroscopy. Ring dye laser fluorescence spectroscopy was used to probe the A «— X system of Ag2 with Doppler-limited resolution. A rotational analysis gave re =2.53118(28) X and r'e =2.65337(31) X. The adiabatic ionisation potentials of Cu2 and CuAg were determined using threshold ionisation spectroscopy (TIS) to be 7.90426(42) eV and 7.78058(54) eV respectively. The dissociation energies of the ion ground states were accordingly determined to be 1.83(08) eV (Cu2) and 1.56(10) eV (CuAg). Rich autoionisation structure in the post-threshold region was observed for both these systems. Electronic excitation in the Group Ib transition metal dimers is discussed in the light of the R2PI, LIF and TTS results.

No description available.

The <SUP>241</SUP>Am photofission and photoneutron cross section, the mean number of prompt neutrons per fission υ* and the delayed neutron yield have been measured between 5 and 10 MeV. The photofission cross section has been measured over a wider range and with better resolution than any previous data. The photoneutron cross section, υ* and the delayed neutron yield had not been measured before. The experiment was peformed using bremsstrahlung radiation produced by the 136 MeV Harwell electron linac HELIOS. Fission neutrons and photoneutrons from the <SUP>241</SUP>Am target were detected by an oil-moderated tank containing fifty-six <SUP>10</SUP>BF<SUB>3</SUB> proportional counters arranged in five concentric rings. For each run, the events associated with each beam burst were recorded as a multiplicity distribution. The end-point energy was cycled between two neighbouring energy points during the run to give difference yields in order to minimise the effects of energy drifting. The data were corrected, fitted and bremsstrahlung unfolded to give both the photofission and photoneutron cross sections as well as υ* and an upper limit for the delayed neutron yield. The results were then compared with other data. An attempt was made to try to explain the structure seen at 6.0 MeV in the photofission cross section of ^241Am in terms of a sub-barrier resonance by fitting a theoretically calculated cross section to the data. Calculations show that there is the possibiity of a resonance in this region, though this would require damping to be less than contemporary theory would suggest. Cross sections were also calculated for the other actinides studied at Harwell with this set-up, namely ^232Th and ^238U.